This invention concerns generally the field of pyridine chemistry, and particularly an improved electrochemical process for preparing N,N'-disubstituted-4,4'-tetrahydrobipyridines through direct reduction of their precursor pyridinium salts in commercially practicable flow cells using high-surface-area cathodes.
An early reported synthesis of these compounds was by direct dimerization of an N-alkylpyridinium salt with sodium amalgum to form N,N'-dialkyl-4,4'-tetrahydrobipyridine. This product was then oxidized to the corresponding N,N'-dialkyl diquaternary salt. Bruno Emmert, "Constitution of the dialkyltetrahydrodipyridyls discovered by A. W. Hofmann," Ber. 52B, 1351-3 (1919); Bruno Emmert, "A radical with quadrivalent nitrogen," Ber. 53B, 370-7 (1920). Another investigation also by Emmert reported the direct electrolysis of N-alkylpyridinium salts to their corresponding N,N'-dialkyl-4,4'-tetrahydrobipyridines in an alkaline solution, also with subsequent oxidation to afford the same N,N'-disubstituted bipyridinium compounds. Bruno Emmert, "Electrolysis of Quaternary Pyridinium and Quinolinium Salts," Ber., 42, 1997-9 (1909).
This electrochemical approach was and is highly appealing as a simple and direct method whereby these tetrahydrobipyridines and their oxidized bipyridinium salts can be obtained while observing moderate conditions and generally without the need for dangerous or noxious substances. Unfortunately, such electrochemical reactions have suffered over the years largely due to problems of commercial practicability. Cell design technology has been slow to advance, and the degree of conversion and yield of targeted products has often been too low for commercial viability.
The field of organic electrochemistry has received renewed attention in the past decade, however, in part as chemical companies have shifted toward more highly functionalized and higher valued products. These N,N'-disubstituted-4,4'-tetrahydrobipyridines are clearly caught up in this resurgence.
For example, in the late 1960's U.S. Pat. No. 3,478,042 to Imperial Chemical Industries Ltd. (ICI) reported an improved method for preparing these compounds by conducting the electrolysis in a glass beaker-type cell using planar electrodes and a diaphragm separator with extraction in situ of the tetrahydropyridine by means of an organic solvent such as diethyl ether, hexane, octane or others added to the catholyte solution. Conversion of the pyridinium salt was reported at 10%, with yield of the targeted tetrahydrobipyridine product reported as equivalent to a current efficiency of 90%. A reported problem with ICI's method, however, has been that conversions cannot be achieved much beyond this 10% level without damaging deposits forming on the electrode surface thereby making continued operation impractical and isolation of the product tedious. Also, an organic extracting agent is expensive, highly flammable, and adds extra unwanted steps to the process. The use of stirred-tank cells also makes such processing uneconomic because productivity is so low.
More recently, U.S. Pat. No. 4,176,020 to Asahi Kasei Kogyo Kabushiki Kaisha (Asahi) reported an improvement of ICI's process utilizing a two- or three-chamber electrolytic vat and aqueous catholyte with no extracting solvent in the catholyte solution. The Asahi patent still requires, however, that extraction of the liquid coming from the cathode chamber take place in a subsequent operation with the organic solvent having been removed to an outside reservoir. This poses continuing problems with Asahi's process as even the external extracting solvent keeps the cost of production high, the necessity remains for separating the aqueous phase cleanly from the organic phase before recycling to the cell, and the linear velocity of electrolyte in the cell is high thereby increasing the pumping and manufacturing costs. The use of flat or planar electrodes is also undesirable as their surfaces must be kept clean and their productivities are low compared to applicant's invention herein.
Regardless of their method of synthesis, once formed these tetrahydrobipyridines exhibit effective properties as oxygen scavengers, as acid-gas scavengers, e.g., of carbon dioxide or hydrogen sulfide, and as anti-corrosion additives. They can also be readily oxidized to diquaternary salts of 4,4'-bipyridines or to 4,4'-bipyridines themselves, many of which exhibit effective herbicidal properties and have gained extensive worldwide use. Principal among these compounds is N,N'-dimethyl-4,4'-bipyridinium dichloride which is commonly referred to by the trademark PARAQUAT.RTM.. For a general report on the synthesis of these diquaternary salts of bipyridine compounds, see L. A. Summers, "The Bipyridinium Herbicides," Academic Press, NY, pp. 69-91, 1980.